As an efficient and energy-saving heat exchange technology, horizontal tube falling film evaporation has a great application prospect in refrigeration and air conditioning. The three-dimensional models of falling film...As an efficient and energy-saving heat exchange technology, horizontal tube falling film evaporation has a great application prospect in refrigeration and air conditioning. The three-dimensional models of falling film flow evaporation outside horizontal single tube and inside evaporator were established, and the accuracy of flow and heat transfer simulation process was verified by comparison. For horizontal single tube, the results showed that total heat transfer coefficient was low and increased with larger spray density and evaporation temperature. The thickness of liquid film outside tube decreased gradually with the increase of tube diameter, and the total heat transfer coefficient of small tube diameter was significantly greater than that of the large tube diameter. The total heat transfer coefficient presented an increasing trend with larger liquid distribution height and density. In addition, the fluctuation of tube axial liquid film thickness distribution decreased with larger liquid distribution density. For evaporator, the results indicated that part of liquid refrigerant was carried into the vapor outlet. The temperature of tube wall and fluid presented a gradually rising trend in vertical downward direction, while tube wall temperature within the same horizontal and transverse row had little difference. The high-temperature zone on the outer wall of heat exchange tube moved towards the inlet and gradually decreased, and the outlet temperature of water in the tube also gradually decreased with the increase of refrigerant spray density. The local heat transfer coefficient of heat exchanger tube in the vertical direction presented a downward trend which was more obvious with the smaller spray density and it was obviously higher located in the middle of upper tube row and both sides of lower tube row for horizontal tube rows.展开更多
For utilization of the residual heat of flue gas to drive the absorption chillers,a lithium-bromide falling film in vertical tube type generator is presented.A mathematical model was developed to simulate the heat and...For utilization of the residual heat of flue gas to drive the absorption chillers,a lithium-bromide falling film in vertical tube type generator is presented.A mathematical model was developed to simulate the heat and mass coupled problem of laminar falling film evaporation in vertical tube.In the model,the factor of mass transfer was taken into account in heat transfer performance calculation.The temperature and concentration fields were calculated.Some tests were conducted for the factors such as Re number,heating flux,the inlet concentration and operating pressure which can affect the heat and mass transfer performance in laminar falling film evaporation.The heat transfer performance is enhanced with the increasing of heat flux.An increasing inlet concentration can weaken the heat transfer performance.The operating pressure hardly affects on heat and mass transfer.The bigger inlet Re number means weaker heat transfer effects and stronger mass transfer.The mass transfer obviously restrains the heat transfer in the falling film solution.The relation between dimensionless heat transfer coefficient and the inlet Re number is obtained.展开更多
The interfacial evaporation of falling water films with wall heating was experimentally studied and analyzed. The results presented in this paper showed that the capillary induced interfacial evaporation played an ...The interfacial evaporation of falling water films with wall heating was experimentally studied and analyzed. The results presented in this paper showed that the capillary induced interfacial evaporation played an important role in heat transfer of a falling liquid film. It would be independent of the wall heat flux and somewhat lower than that without wall heating for impure fluids such as water air system. The thermodynamic analysis conducted gave a theoretical basis for the experimental observations. The effective capillary radius was correlated with the mass flow rate. The experimental results and analysis showed that the interfacial evaporation should be taken into account in the study of falling liquid film heat transfer.展开更多
Analysis of experimental data and estimation of the order of magnitude for interfacial mass diffusion have demonstrated that considerable excess evaporation exists on the free interface of falling liquid film, and tha...Analysis of experimental data and estimation of the order of magnitude for interfacial mass diffusion have demonstrated that considerable excess evaporation exists on the free interface of falling liquid film, and that the capillary pressure caused by surface tension is the driving force of this excess interfacial evaporation, which we called the “capillarity-induced interfacial evaporation”. By correlating the experimental data, an empirical expression of the effective capillary radius, r\-e, is obtained with which the evaporative rate formula we derived and reported previously has been modified to improve the prediction of the critical heat flux for film breakdown. Comparisons with the available predicting models show that our modified equation can predict the experimental results with much lower relative deviation.展开更多
水平管降膜蒸发器是广泛应用于海水淡化和化学工业等方面的换热设备。建立了三维数值模型,采用VOF(volume of fluid)方法结合UDF(user defined function)程序对水平管外降膜蒸发过程的换热特性进行了数值模拟研究,深入研究了光管和强化...水平管降膜蒸发器是广泛应用于海水淡化和化学工业等方面的换热设备。建立了三维数值模型,采用VOF(volume of fluid)方法结合UDF(user defined function)程序对水平管外降膜蒸发过程的换热特性进行了数值模拟研究,深入研究了光管和强化管针对不同管间距、喷淋密度、蒸发温度和管径对液膜厚度以及平均传热系数的影响。模拟结果表明:在相同条件下,强化管的平均传热系数要明显高于光管,可见基于仿生学原理优化换热表面结构能有效提高水平管外降膜蒸发的换热效率。光管和强化管的平均传热系数随着蒸发温度的升高都呈上升趋势,随喷淋密度的增加呈先上升后减少的趋势。光管和强化管的平均换热系数随管间距的增大而增大,随喷淋密度的增加呈先增加后减少的趋势;换热管的平均传热系数随着蒸发温度升高而升高,随喷淋密度的增加呈先上升后下降的趋势。展开更多
基金financially supported by National Natural Science Foundation of China (No. 52006031)international cooperation project of China Manned Space Program (6903001173)。
文摘As an efficient and energy-saving heat exchange technology, horizontal tube falling film evaporation has a great application prospect in refrigeration and air conditioning. The three-dimensional models of falling film flow evaporation outside horizontal single tube and inside evaporator were established, and the accuracy of flow and heat transfer simulation process was verified by comparison. For horizontal single tube, the results showed that total heat transfer coefficient was low and increased with larger spray density and evaporation temperature. The thickness of liquid film outside tube decreased gradually with the increase of tube diameter, and the total heat transfer coefficient of small tube diameter was significantly greater than that of the large tube diameter. The total heat transfer coefficient presented an increasing trend with larger liquid distribution height and density. In addition, the fluctuation of tube axial liquid film thickness distribution decreased with larger liquid distribution density. For evaporator, the results indicated that part of liquid refrigerant was carried into the vapor outlet. The temperature of tube wall and fluid presented a gradually rising trend in vertical downward direction, while tube wall temperature within the same horizontal and transverse row had little difference. The high-temperature zone on the outer wall of heat exchange tube moved towards the inlet and gradually decreased, and the outlet temperature of water in the tube also gradually decreased with the increase of refrigerant spray density. The local heat transfer coefficient of heat exchanger tube in the vertical direction presented a downward trend which was more obvious with the smaller spray density and it was obviously higher located in the middle of upper tube row and both sides of lower tube row for horizontal tube rows.
文摘For utilization of the residual heat of flue gas to drive the absorption chillers,a lithium-bromide falling film in vertical tube type generator is presented.A mathematical model was developed to simulate the heat and mass coupled problem of laminar falling film evaporation in vertical tube.In the model,the factor of mass transfer was taken into account in heat transfer performance calculation.The temperature and concentration fields were calculated.Some tests were conducted for the factors such as Re number,heating flux,the inlet concentration and operating pressure which can affect the heat and mass transfer performance in laminar falling film evaporation.The heat transfer performance is enhanced with the increasing of heat flux.An increasing inlet concentration can weaken the heat transfer performance.The operating pressure hardly affects on heat and mass transfer.The bigger inlet Re number means weaker heat transfer effects and stronger mass transfer.The mass transfer obviously restrains the heat transfer in the falling film solution.The relation between dimensionless heat transfer coefficient and the inlet Re number is obtained.
基金the National Natural Science Foundationof China (No.5 9995 5 5 0 - 3)
文摘The interfacial evaporation of falling water films with wall heating was experimentally studied and analyzed. The results presented in this paper showed that the capillary induced interfacial evaporation played an important role in heat transfer of a falling liquid film. It would be independent of the wall heat flux and somewhat lower than that without wall heating for impure fluids such as water air system. The thermodynamic analysis conducted gave a theoretical basis for the experimental observations. The effective capillary radius was correlated with the mass flow rate. The experimental results and analysis showed that the interfacial evaporation should be taken into account in the study of falling liquid film heat transfer.
基金the Science Funds of Ministry of Education of China and the National Natural Science Foundation of China (Grant No. 59995550-3)
文摘Analysis of experimental data and estimation of the order of magnitude for interfacial mass diffusion have demonstrated that considerable excess evaporation exists on the free interface of falling liquid film, and that the capillary pressure caused by surface tension is the driving force of this excess interfacial evaporation, which we called the “capillarity-induced interfacial evaporation”. By correlating the experimental data, an empirical expression of the effective capillary radius, r\-e, is obtained with which the evaporative rate formula we derived and reported previously has been modified to improve the prediction of the critical heat flux for film breakdown. Comparisons with the available predicting models show that our modified equation can predict the experimental results with much lower relative deviation.
文摘水平管降膜蒸发器是广泛应用于海水淡化和化学工业等方面的换热设备。建立了三维数值模型,采用VOF(volume of fluid)方法结合UDF(user defined function)程序对水平管外降膜蒸发过程的换热特性进行了数值模拟研究,深入研究了光管和强化管针对不同管间距、喷淋密度、蒸发温度和管径对液膜厚度以及平均传热系数的影响。模拟结果表明:在相同条件下,强化管的平均传热系数要明显高于光管,可见基于仿生学原理优化换热表面结构能有效提高水平管外降膜蒸发的换热效率。光管和强化管的平均传热系数随着蒸发温度的升高都呈上升趋势,随喷淋密度的增加呈先上升后减少的趋势。光管和强化管的平均换热系数随管间距的增大而增大,随喷淋密度的增加呈先增加后减少的趋势;换热管的平均传热系数随着蒸发温度升高而升高,随喷淋密度的增加呈先上升后下降的趋势。
